1. Field of the Invention
This invention relates to interferometry and more particularly to interferometric devices used for measuring relative distances between two objects.
2. Description of the Related Art
Interferometry is the area of science that uses interference patterns to evaluate data such as the distance between two objects. In radio astronomy, interference patterns are generated by adding signals radiated from a star or the like. A phase difference is generated by providing the two signals from spatially-separated antennas. The longer the base line, the higher the resolving power of the radio antenna.
Interferometry is used to measure short distances using laser light that is reflected and refracted in certain predictable ways. The famous Michaelson-Morley experiment is one example of this, and such devices are used in a similar fashion today. In a Michaelson interferometer, when the displacement mirror is moved, the interference pattern cyclically changes, showing the alternating constructive and destructive interference between the two light wave fronts. The cyclic nature of the interference pattern indicates the relative phase difference between the two light wave fronts. This is dependent upon the wavelength of light with smaller wavelengths of light providing more accurate measurements.
Present-day displacement-measuring interferometers generally use two orthogonal polarizations corresponding to two interfering light beams. Leakage occurs across the two otherwise independent light beams, resulting in a phenomena called polarization leakage. When the polarization leaks from one light beam to another, the phase differences between the two are changed and the phase measurement is contaminated. The errors arising in the phase measurement are periodic and non-linear errors. From most commercial interferometers, this non-linear error is approximately 1-10 nm (10xe2x88x929 m). Additionally, such interferometers are subject to thermal errors arising from temperature changes occurring in the apparatus. The thermal sensitivity of such present-day devices gives rise to errors of approximately 100 nm per degree Kelvin (xc2x0 K.). These thermal errors arise from non-compensated paths that the light beams take through the optics of the devices.
As present-day interferometers suffer from certain flaws and errors in measurement arising from the construction of such interferometers, it would be an advance in the art to provide an interferometer that does not generate polarization leakage and that also avoids errors arising from thermal sensitivity.
The interferometer of the present invention uses two stable collimated laser beams that are slightly different in frequency, i.e., f0 for one beam and f0+xcex94f for the other beam. The f0 wavefront is split into two or more symmetric sections with a reference device such as a truncated corner cube or a retro-mirror with holes. The wavefront portions that are reflected from the reference device then serve as reference signals. The remaining wavefront portions are directed to the measurement target(s) which retro-reflect the measurement beams back to the interferometer. All these portions will then be mixed (interfered) with the f0+xcex94f beam which serves as a local oscillator. These heterodyne fringes are then separated with truncated mirrors and focused into photo-detectors.
The phase difference (xcex94xcfx86) between the measurement signals and reference signals is measured with phase meters. The displacement (xcex94L) between the target(s) and the reference device is related to the phase difference: xcex94L=xcexxcex94xcfx861/(4xcfx80).
Because the reference signals are derived from the same wavefront as the measurement signals, the optical path length change in the optical elements consisting of the interferometer is common-mode. Thus the interferometer is insensitive to soak temperature changes.
There is no polarization leakage in this interferometer. In addition, angle-polished fibers are used in the construction of the collimators to minimize the back-reflection from the fiber tip. The beam splitters have a small wedge so that the ghost reflection from the back surface will not contaminate the heterodyne signals. These measures greatly reduce the periodic nonlinear error of the interferometer.
The present invention provides measurements within accuracy of approximately 20 picometers (10xe2x88x9212 m). Additionally, the present invention provides an a thermalized structure to provide a device that is orders of magnitude less sensitive to thermal shifts than prior interferometers. The present invention also provides greater accuracy. A wavefront split of a heterodyne light signal is used as the means by which displacement measurements are made. This is in contrast to existing devices that use an amplitude split rather than a wavefront split in order to make such displacement measurements. By using a wavefront split, the present invention exhibits extremely low self-interference and avoids the problems of polarization leakage leading to the higher accuracy in measurements. Additionally, each light path is generally the same, which allows errors from thermal sensitivity to cancel out or be significantly diminished.
When the second optical target, which is the one subject to relative displacement, is moved with respect to the first target, the change in distance (xcex94L) is equal to the change in the phase (xcex94xcfx86) times the wavelength of the light (xcex) divided by four pi (4xcfx80) (xcex94L=xcex94xcfx86(xcex/4xcfx80). Two embodiments are currently known, however other embodiments may be developed in the future.
It is an object of the present invention to provide more accurate measurements for relative displacement.
It is yet another object of the present invention to provide better measurements of relative displacement by using interferometry.
It is yet another object of the present invention to provide a better relative-displacement interferometer that is generally athermalized and not as subject to changes in thermal conditions.
It is yet another object of the present invention to provide an interferometer that is less subject to polarization leakage in order to provide better phase difference measurements.